I have a proxy Server and a PBX server that both have their own router as a gateway. Nevertheless, the company acquired an additional more stable ISP and now I have three Internet connections.

Is there a way I can configure the proxy to use both at the same time? Is there an approach to use two gateway with one proxy?


Since i know i provide too few details here is a more describe scenario:

I have two server one is the PBX and the other is DNS/Proxy/IpTable so for voice and calls peoples goes to PBX wich has its own gateway, for Internet data access users request are handle by the second server. Nevertheless our ISP have been unstable between now and then and i need to provide another internet gateway. I really like the idea of a multiple wan router but i've also been reding that i can by adding my server DNS/Proxy a third NIC.

Aditional Info:

Current routers:

  • 2 small home offices linksys router (one for voice one for data)
  • IT have expirence enough knowleged to handle the configuratoin and actitude to learned this in the proper manner that's why he is asking on serverfault
  • Load balancing it would be great to implement.


  • can you confirm that the DNS/Proxy/IpTables box is integrated on one device, which runs linux? Apr 27, 2011 at 23:14
  • it is and running with CentOS 5....
    – Necronet
    Apr 28, 2011 at 2:02

2 Answers 2


There is not enough detail in this question to provide a complete solution; however there are some general principles I can share which you might be able to research on your own and apply.

The relevant details include:

  • Router / Firewall models
  • Resilience & recovery time in the face of link / equipment failure
  • Topology
  • Future plans for expansion
  • Technical abilities of your IT staff
  • How important load-balancing is against the cost of doing it (Additional: Opex, Capex, migration downtime, complexity)

You should also understand that saying you want to load-balance is like walking into a grocery store and telling the clerk you want to buy some food. The first question will be 'what kind of food'? In the same manner, understand that at a minimum, you must consider the cases of outbound (i.e. from your servers to the internet) and inbound (from internet to your servers) load-balancing separately.

To be honest, if you ask this question with so little contextual information, experience tells me that you're probably getting in over your head. Furthermore, the time to design a load-balancing solution is before the company signs the agreements to purchase additional bandwidth. I'm not saying this as a slight against you, but it should be taken as a precautionary note; load-balancing solutions are complicated, and the requirements need to be fully considered, including what happens when things fail, or components need to be added / expanded. If someone else decided to buy bandwidth and then handed you the project as an afterthought, I co-miserate... I've been there too.

Outbound Load Balancing

There are several ways to influence outbound load-balancing:

  • Equal Cost Multi-Path (ECMP), which means you set up equal-cost paths outbound to the internet. This is easier said than done... most of the time, topology or equipment issues get in the way of doing this across seperate ISPs, but it is possible.
  • Unique NAT overloads per-ISP with the Firewall doing the balancing
  • Dedicated load-balancing hardware (like an F5)
  • First-hop L2 technologies... creative use of GLBP / HSRP / VRRP
  • Partial routes to the internet... via BGP filtering or local-pref... with traffic shifting between your eBGP routers across dedicated high-bandwidth iBGP links
  • Dual links to the same ISP router

Inbound Load-balancing

There are several ways to influence inbound load-balancing:

  • Dual links to the same ISP router
  • Unique NAT overloads per-ISP with the Firewall doing the balancing
  • Multiple exits to the internet via multiple ISPs, with eBGP policies that nudge traffic one way or another

If you can post more details of the requirements and environment, you might be able to get a good solution on ServerFault... then again, you might get a bunch of people who hope for a little glory insisting that their way is best; meanwhile situational factors unique to your corporate environment sabotage the implementation of said recommendation(s).


Assuming you have a linux machine, I would take a look at the possibilities available in netfilter... one option is shorewall. They have an official howto and another howto, which cover the basics of load-balancing across multiple ISPs through a linux bastion-host firewall, but your situation is more complicated since you have the IP PBX involved.

I will defer to someone who has actually done this to post detailed suggestions.

Shorewall use case

Shorewall use case


If you do not know how to build one, you will need to purchase a network device (router) capable of load balancing between multiple ISPs.

With inbound traffic, one solution would be to use a dynamic routing protocol such as BGP.

With outbound traffic, a solution would be to failover a VIP between multiple devices using a protocol such as HSRP.

If you do not have the knowledge or resources available to build a more robust solution, you could seek a router capable of handling multiple WAN connections. You will likely find affordable SOHO equipment able to serve your needs. It is important to realize this router is going to be a single point of failure unless you introduce redundancy.

Another option would be with a single server, you could potentially introduce multiple routes with different metrics.

  • i like the multple WAN connection solution!!
    – Necronet
    Apr 27, 2011 at 22:43

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